The invention relates to a solenoidal magnet, without a core, having one or more coils, whose technological structure is derived from that of a conventional Bitter coil; the invention relates more particularly to improvements for improving the homogeneity of the magnetic field generated by such a type of magnet.
Bitter coils are well known for producing intense magnetic fields. In theory, the structure proposed by Bitter is a winding formed of metal annular disks split so as to form as many turns and connected together so as to define a substantially helical winding with flat turns. The stack of disks is held in position by a plurality of tie rods. This structure is advantageous for it allows efficient cooling of the magnet by forming holes in the rings (and in the insulators separating these disks), these holes being disposed in the same configuration from one disk to another so as to form an assembly of channels parallel to the axis of the coil, in which a cooling fluid flows, for example deionized water, kerosene or oil.
The invention proposes perfecting such a type of magnet so that the magnetic field generated in a sphere of interest of prescribed radius, whose center merges with the center of symmetry of this magnet, has very good homogeneity. A privileged field of application of the invention is in fact that of image formation by nuclear magnetic resonance (NMR) where it is necessary to have a relatively high magnetic field (0.15 to 1.5 teslas with a very high homogeneity, of the order of 1 to 10 parts per million (ppm). With a sufficiently long Bitter coil, a certain homogeneity may be obtained about the center of symmetry of this coil. This homogeneity will be more easily attained and with a more compact structure either by varying the thickness of the disks along the axis of the coils or by aligning several Bitter coils along a common axis, the lengths of the coils and spacing therebetween being chosen so as to obtain the required homogeneity.
These solutions are described in other patent applications filed by the applicant. The improvements of the invention apply not only to the magnet with a single coil but also to a magnet with several aligned coils.
There may in fact exist other structural causes of inhomogeneity of the magnetic field generator or causes for the disturbance of this magnetic field.
Among these the way in which the current is applied to the magnet must be taken particularly into consideration. In fact, if the connection between the supply source and the magnet is formed conventionally by means of two conductors connected respectively to the axial ends of the magnet, field disturbances generated by these conductors may degrade the homogeneity of the field in the sphere of interest.
The purpose of the invention is first of all to overcome this problem.